Recently the American Association for the Study of Liver Disease stated that "...Overall, patients with non-alcoholic fatty liver disease (NAFLD) have reduced long-term survival, with liver disease accounting for about 10% of overall mortality.... Clearly it appears that NAFLD will become a more prevalent problem in coming years..." This indicates that NAFLD will become a major public health issue in the near future. However, to date, therapeutic options for this disease are limited. In this regard, it is imperative to further understand the molecular mechanisms that modulate the development of liver steatosis. We observed that the protein Deleted in Breast Cancer 1 (DBC1) plays a crucial role in the development of experimental liver steatosis. In particular we found that mice knocked out for DBC1 are resistant to high fat diet-induced liver steatosis, while the systemic metabolic syndrome was not ameliorated. This suggests that DBC1 controls intrinsic hepatic mechanisms involved in the development of liver steatosis. To date very little is known about the physiological and biochemical roles of DBC1, however recent studies indicate that DBC1 binds and inhibits the enzyme SIRT1. SIRT1 is a NAD-dependent deacetylase that controls metabolism, ageing and longevity. In fact, it has been proposed that SIRT1 is a metabolic master switch. Pharmacological activation of SIRT1 protects against liver steatosis, at least in part, via stimulation of the AMP kinase (AMPK) that subsequently decreases lipogenesis and increases fatty acid oxidation. Although the metabolic actions of SIRT1 are the subject of intense investigation;much less is known about regulation of SIRT1. In fact, the mechanisms that modulate SIRT1 expression and activity remain elusive. In this regard, the characterization of DBC1 as an endogenous inhibitor of SIRT1, and the determination of the mechanisms that regulate the DBC1-SIRT1 interaction are of extreme importance. It is possible that modulation of DBC1-SIRT1 interaction plays a key role in the regulation of SIRT1 function during different metabolic conditions, including diet-induced steatosis. An increase in interaction between DBC1 and SIRT1 may also provide the molecular basis for the decrease in SIRT1 activity reported in animal models of diet-induced liver steatosis. Thus, our Central Hypothesis is that DBC1 plays a crucial role in the development of liver steatosis by direct binding and inhibiting SIRT1, with the subsequent modulation of downstream effects of SIRT1 on hepatic lipogenesis and 2-oxidation via AMPK. To test this hypothesis, the following specific aims will be addressed: Aim 1. Study the role of SIRT1 in DBC1-mediated regulation of liver steatosis. Aim 2. Characterize the role of DBC1 in the regulation of SIRT1 activity during different caloric loads and determine the molecular mechanisms that regulate the DBC1-SIRT1 interaction. Aim 3. Determine the biochemical mechanisms, downstream from SIRT1, by which DBC1 regulates the development of liver steatosis: role of AMPK. Collectively, these studies will lead to an in depth and extremely novel understanding of the physiological roles of DBC1 as a regulator of SIRT1, liver fat metabolism and hepatic steatosis. PUBLIC HEALTH RELEVANCE: Non alcoholic fatty liver disease (NAFLD) is a major health problem worldwide. As recently stated by members of the future trend meeting of the American Association for the Study of Liver Disease, "...Overall, patients with NAFLD have reduced long-term survival, with liver disease accounting for about 10% of overall mortality, surpassed only by malignancy and cardiovascular deaths. Clearly it appears that NAFLD will become a more prevalent problem in coming years..." ). These alarming data and predictions clearly show that it is imperative to further understand the molecular mechanisms that modulate the development of liver steatosis.